1. Field of the Invention
Aspects of the present invention relate to a fuel processor of a fuel cell system, and more particularly, to a reformer included in a fuel processor, and a method of controlling the same.
2. Description of the Related Art
A fuel cell is a generator of electricity that changes the chemical energy of a fuel into electrical energy, through a chemical reaction. A fuel cell can continuously generate electricity as long as the fuel is supplied. Fuel cell systems can be broadly divided into fuel cell systems that use liquid hydrogen, and fuel cell systems that use hydrogen gas. The fuel cell systems that use hydrogen gas include fuel cell stacks and fuel processors. The fuel cell stacks have a structure in which a few to a few tens of unit cells, each including a membrane electrode assembly (MEA), and a separator, are stacked.
FIG. 1 is a schematic diagram showing a configuration of a conventional fuel cell system.
Referring to FIG. 1, a fuel, that includes hydrogen atoms, is reformed into hydrogen gas in a fuel processor, and the hydrogen gas is supplied to a fuel cell stack. In the fuel cell stack, the hydrogen gas is electrochemically reacted with oxygen to generate electrical energy.
The fuel processor includes a desulfurizer and a hydrogen generation apparatus. The hydrogen generation apparatus includes a reformer and a shift reactor. The desulfurizer removes sulfur from the fuel so that catalysts, in the reformer and the shift reactor, are not poisoned by sulfur compounds.
Hydrogen gas is generated from the hydrocarbons in the reformer, but in addition to the hydrogen gas, carbon dioxide (CO2) and carbon monoxide (CO), are also produced. However, CO acts as a poison to the catalysts used on electrodes of the fuel cell stack. Therefore, the hydrogen gas generated in the reformer is not directly supplied to the fuel cell stack, but rather is supplied after the CO is removed by the shift reactor. Conventionally, the hydrogen gas that has passed through the shift reactor has a CO content of 10 ppm or less.
FIG. 2 is a cross-sectional view illustrating a conventional reformer. FIG. 3 is a graph showing the temperature distribution in the reformer of FIG. 2, at different locations of a reforming catalyst. In FIG. 3, the temperature distributions in the reformer are compared at different positions, in a combustion chamber thereof, when loads of 100% and 25% are applied to a burner.
Referring to FIG. 2, a conventional reformer 10 includes a burner 15 that can eject one large flame 25 into a combustion chamber 11, which is disposed inside a pipe-shaped reforming catalyst 20. When a combustion fuel, composed of methane CH4 and air, is ignited by ejecting the combustion fuel into the combustion chamber 11, via the burner 15, the combustion fuel is combusted, and a flame 25 is generated, heating the reforming catalyst 20. Thus, a hydrogen generation reaction occurs in the reforming catalyst 20.
A fuel cell system may operate at 100% of a designed power production capacity (load), or may operate at less than 100% of the designed capacity, according to power consumption of electrical equipment electrically connected to the fuel cell system. When the fuel cell system is operated with a load that is less than 100% of the designed capacity, the burner 15 of the reformer 10 is also operated at a reduced load. More specifically, the loads to the burner 15, and the reformer 10, are proportional to the load to the fuel cell system 100 as a whole.
Referring to FIG. 3, different portions H, of the reforming catalyst 20, in the reformer 10 of FIG. 2, have different temperatures. More specifically, a central portion B, of the reforming catalyst 20, which is closer to the flame 25, has a relatively high temperature, and a lower and upper portions A and C of the reforming catalyst 20, which are relatively farther from the flame 25, have relatively lower temperatures. Also, the size of the flame 25 is larger when a load to the burner 15 is 100%, than when the load to the burner 15 is 25%. Thus, the overall temperature of the reforming catalyst 20, when a load to the burner 15 is 100%, is higher than when the load to the burner 15 is 25%.
The hydrogen generation reaction, on the reforming catalyst 20, is an endothermic reaction, and the hydrogen generation reaction is conducted at a temperature of approximately 700° C., or more. In the reformer 10, there are large temperature differences, according to the height of the reforming catalyst 20. The temperature of the central portion B can be maintained at 700° C., or more, regardless of the load to the burner 15, but it is difficult to maintain the temperatures of the lower and upper portions A and C at 700° C., or more. In particular, it is particularly difficult to maintain the temperature of 700° C. at the lower and upper portions A and C, when the load to the burner 15 is small. Accordingly, there is a problem that, although the reforming catalysts at the lower and upper portions A and C, of the reforming catalyst 20, are not completely consumed, all of the reforming catalyst 20 must be replaced, due to the exhaustion of the reforming catalysts in the central portion B.